Amido-carboxylic acids are industrial chemical intermediates for the preparation of many chemicals used in commerce. Amido-carboxylic acids are prepared by reacting a lactam with a carboxylic acid. Amido-carboxylic acids are also prepared by reacting a carboxylic acid, carboxylic acid chloride, carboxylic acid anhydride or carboxylic acid ester with an amino-carboxylic acid. These processes which form amido-carboxylic acids are referred to as amidation reactions.
It is known to convert lactams by hydrolysis into the corresponding amino-carboxylic acids in the presence of hydrolysis promoting reagents such as hydrochloric acid. However, pure amino-carboxylic acids are not directly obtained. In the case where hydrochloric acid is used as the promoting reagent, the amino-carboxylic acid hydrochloride is obtained and the separation of the free amino-carboxylic acid is cumbersome and expensive.
U. S. Pat. No. 2,453,234 discloses a process for preparing an amino-carboxylic acid by hydrolyzing a lactam by means of at least 10 moles of water per mole of lactam to produce an amino-carboxylic acid. Great Britain Pat. No. 648,889 discloses a process for preparing amino-carboxylic acids by heating aliphatic or cyclo-aliphatic lactams in the presence of more than 20 moles of water per mole of lactam. U.S. Pat. No. 2,956,068 discloses a process for preparing amido-carboxylic acids by reacting a lactam with a free carboxylic acid in the presence of catalytic amounts of water. The reaction product is obtained as a solid crystal mass which is subsequently suspended in water and neutralized. The reaction product contains significant quantities of oligomeric amido-carboxylic acids which are made up of a carboxylic acid chain and two or more repeating amino-carboxylic acid units. These oligomeric amido-carboxylic acids are undesirable for many applications.
U.S. patent application Ser. No. 08/228,611 discloses an aqueous process for preparing amido-carboxylic acids wherein lactam hydrolysis, carboxylic acid ester hydrolysis, and amidation reactions are conducted simultaneously. The process involves heating water, a lactam or an amino-carboxylic acid, and a carboxylic acid or ester to form a reaction mixture which is cooled to obtain a two phase system containing an aqueous phase and an organic phase. The organic phase containing the amido-carboxylic acid is separated from the aqueous phase. The reaction product contains significant quantities of oligomeric amido-carboxylic acids which are made up of a carboxylic acid chain and two or more repeating amino-carboxylic acid units. These oligomeric amido-carboxylic acids are undesirable for many applications.
Waste materials are also generated when monomeric amino-carboxylic acids are polymerized. For example, the manufacture of nylon 6 polymer (polycaproamide), produces about 8-10% low molecular weight oligomers (extractables) as waste. The manufacture of nylon 6 fiber generates about 5-12% solid waste. According to the Indian Journal of Fibre & Textile Research, Vol. 16, March 1991, pp. 46-51, India generates about 7000-8000 metric tonnes of solid waste (nylon 6) every year. Thus, it is desirable for economic and environmental reasons to recycle the extractables and the solid waste.
U.S. Pat. No. 4,605,762 discloses a process for depolymerizing condensation polymers. The process involves subjecting condensation polymer waste material to aqueous hydrolysis in a hydrolysis zone, introducing high pressure steam into the hydrolysis zone below the level where the polymer waste material is introduced, and withdrawing an aqueous solution of the products of the hydrolysis reaction from an upper portion of the hydrolysis zone.
U.S. Pat. No. 5,169,870 discloses a process for recovering caprolactam from nylon 6 carpet. The process involves breaking the carpet down into scrap containing nylon 6 and non-nylon 6 backing materials. The scrap is fed into a depolymerizing reactor where the scrap is subjected to a depolymerization catalyst, temperatures of at least the melting point of nylon 6 and superheated steam to produce a caprolactam containing distillate. The caprolactam in the distillate is separated from other volatiles.